The disclosure of Japanese Patent Application No. 2002-143393, including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of Invention
The invention relates to air-fuel ratio control system and method for an internal combustion engine including an exhaust passage and a catalyst disposed in the same passage.
2. Description of Related Art
There is known an internal combustion engine including an exhaust passage and a three-way catalyst disposed in the same passage for controlling an exhaust gas. A three-way catalyst (hereinafter will be simply referred to as a xe2x80x9ccatalystxe2x80x9d where appropriate) is capable of storing oxygen. More specifically, when the air-fuel ratio of the exhaust gas flowing into the catalyst is rich, the catalyst oxidizes unburned components, such as HC and CO, using oxygen stored therein. In contrast, when that air-fuel ratio is lean, the catalyst reduces nitrogen oxides (NOx) and stores the oxygen removed from those nitrogen oxides reduced. Having such capabilities, three way catalysts can be effectively used for purifying an exhaust gas by controlling unburned components and nitrogen oxides, which tend to increase as the air-fuel ratio in the internal combustion engine deviates from the stoichiometric air-fuel ratio. By the way, the purification capacity of such a three-way catalyst becomes larger as the maximum storable oxygen thereof increases.
The maximum storable oxygen amount changes depending upon the state of the catalyst that physically degrades with use. Therefore, it is possible to determine the degradation of the catalyst by estimating its maximum storable oxygen amount.
The catalyst degradation determination system disclosed in Japanese Laid-opened Patent Application No. 5-133264 employs the above concept for determining the degree of the catalyst degradation, as will be described in the following. That is, in this system, the air-fuel ratio in the internal combustion engine, which is namely the air-fuel ratio upstream of the catalyst, is changed from a predetermined rich air-fuel ratio to a predetermined lean air-fuel ratio, or from a predetermined lean air-fuel ratio to a predetermined rich air-fuel ratio. Then, the maximum amount of oxygen storable in the catalyst (hereinafter will be referred to as the xe2x80x9cmaximum storable oxygen amount of the catalystxe2x80x9d) is estimated from a change in the output of an air-fuel ratio sensor disposed downstream of the catalyst during the above shift of the air-fuel ratio upstream of the catalyst, and the degradation degree of the catalyst is determined based on the maximum storable oxygen amount estimated.
More specifically, in the above system, the air-fuel ratio upstream of the catalyst is made equal to the predetermined rich air-fuel ratio by a so-called open-loop control so that the amount of oxygen stored in the catalyst becomes zero, after which the air-fuel ratio upstream of the catalyst is made equal to the predetermined lean air-fuel ratio by another open-loop control upon detecting via the air-fuel ratio sensor disposed downstream of the catalyst that the air-fuel ratio downstream of the catalyst has become rich. Each open-loop control is to perform only a feed-forward control on the air-fuel ratio of an air-fuel mixture (gas) to be supplied into the internal combustion engine (i.e., the air-fuel ratio upstream of the catalyst), rather than performing it in combination with a feedback control on the air-fuel ratio actually detected in the gas ejected from the internal combustion engine.
Subsequently, the system obtains the amount of oxygen entering the catalyst from when the air-fuel ratio upstream of the catalyst is switched to the predetermined lean air-fuel ratio to when it is detected via the air-fuel ratio sensor disposed downstream of the catalyst that the air-fuel ratio downstream of the catalyst has become lean as a result of the oxygen amount stored in the catalyst having reached its full capacity. Eventually, the obtained oxygen amount is estimated as the maximum storable oxygen amount of the catalyst.
Alternatively, the air-fuel ratio upstream of the catalyst is made equal to the predetermined lean air-fuel ratio by an open-loop control so that oxygen is stored in the catalyst to its full capacity, after which the air-fuel ratio upstream of the catalyst is made equal to the predetermined rich air-fuel ratio by another open-loop control upon detecting via the air-fuel ratio sensor disposed downstream of the catalyst that the air-fuel ratio downstream of the catalyst has become lean. Subsequently, the system obtains the amount of oxygen consumed in the catalyst from when the air-fuel ratio upstream of the catalyst is switched to the predetermined rich air-fuel ratio to when it is detected via the air-fuel ratio sensor disposed downstream of the catalyst that the air-fuel ratio downstream of the catalyst has become rich as a result of the oxygen stored in the catalyst having been completely used up. Eventually, the obtained oxygen amount is estimated as the maximum storable oxygen amount of the catalyst.
With such a conventional system, however, because the air-fuel ratio upstream of the catalyst is controlled to the predetermined rich or lean air-fuel ratio by the open loop control described above, it is difficult to preserve sufficient accuracy in controlling the air-fuel ratio due to a change in the engine operation state, a variation among individual engines, and the like. Therefore, there is a possibility that an air-fuel mixture of an excessive rich or lean air-fuel ratio be supplied into the internal combustion engine, which may cause problems like a deterioration in the drivability of the vehicle.
However, such a problem with the above conventional system may be resolved by performing a feedback control on the air-fuel ratio upstream of the catalyst during the calculation of the maximum storable oxygen amount of the catalyst in the following manner. That is, a fuel injection amount required for achieving the stoichiometric air-fuel ratio upstream of the catalyst is predetermined as a basic injection amount. Then, when the target value of the air-fuel ratio upstream of the catalyst is set to the predetermined rich or lean air-fuel ratio aforementioned, a feedback correction amount is determined by executing a so-called PI or PID control on the deviation between the actual air-fuel ratio detected by the air-fuel ratio sensor disposed upstream of the catalyst and the target air-fuel ratio, and the basic injection amount is corrected using the feedback correction amount determined.
However, during such a feedback control, a so-called control delay unavoidably occurs. Therefore, due to such a control delay, the emission may increase during the above feedback control executed for estimating the maximum storable oxygen amount of the catalyst.
That is, referring to the timechart shown in FIG. 17, the target air-fuel ratio is switched to the predetermined lean air-fuel ratio when the air-fuel ratio downstream of the catalyst indicating a lean air-fuel ratio changes to indicate a rich air fuel ratio at the time t10. However, the air-fuel ratio of the gas flowing into the catalyst remains rich until the time t20 due to a delay in the feedback control. Because the stored oxygen amount is zero at this time, unburned components, such as CO, are not reduced in the catalyst.
Likewise, the target air-fuel ratio is switched to the predetermined rich air-fuel ratio when the air-fuel ratio downstream of the catalyst indicating a rich air-fuel ratio changes to indicate a lean air-fuel ratio at the time t30. However, the air-fuel ratio of the gas flowing into the catalyst remains lean until the time t40 due to a delay in the feedback control. Because the stored oxygen amount of the catalyst is equal to its maximum storable oxygen amount at this time, NOx contained in the gas is not reduced.
In view of the above situation, the invention has been made to provide the following air-fuel mixture control system and method for an internal combustion engine including an exhaust passage and a catalyst disposed in the same passage.
An air-fuel ratio control system according to one aspect of the invention includes: intake volume obtaining means for obtaining a value corresponding to an intake volume of the internal combustion engine; upstream-side air-fuel ratio detecting means for detecting an air-fuel ratio upstream of the catalyst; downstream-side air-fuel ratio detecting means for detecting an air-fuel ratio downstream of the catalyst; target air-fuel ratio setting means which sets a target air-fuel ratio to a predetermined rich air-fuel ratio that is rich of a stoichiometric air-fuel ratio when it is determined that the air-fuel ratio downstream of the catalyst has changed from a rich air-fuel ratio to a lean air-fuel ratio, and which sets the target air-fuel ratio to a predetermined lean air-fuel ratio that is lean of the stoichiometric air-fuel ratio when it is determined that the air-fuel ratio downstream of the catalyst has changed from a lean air-fuel ratio to a rich air-fuel ratio; fuel supply amount calculating means which calculates, as a feed-forward fuel supply amount, a fuel amount required for an air-fuel ratio of an air-fuel mixture, to be supplied to the internal combustion engine, to become rich of the stoichiometric air-fuel ratio on the basis of at least the detected intake volume when the target air-fuel ratio is set to the predetermined rich air-fuel ratio; which calculates, as the feed-forward fuel supply amount, a fuel amount required for the air-fuel ratio of the air-fuel mixture, to be supplied to the internal combustion engine, to become lean of the stoichiometric air-fuel ratio on the basis of at least the detected intake volume when the target air-fuel ratio is set to the predetermined lean air-fuel ratio; which calculates a feedback correction amount based on the air-fuel ratio detected by the upstream-side air-fuel ratio detecting means and the target air-fuel ratio such that the air-fuel ratio detected by the upstream-side air-fuel ratio detecting means matches the target air-fuel ratio; and which determines the feed-forward fuel supply amount corrected by the feedback correction amount as a final fuel supply amount; and fuel supplying means for supplying the internal combustion engine with the same amount of fuel as the final fuel supply amount.
According to this construction, the target air-fuel ratio is set to the predetermined rich air-fuel ratio, a specific air-fuel ratio rich of the stoichiometric air-fuel ratio, when it is determined that the air-fuel ratio downstream of the catalyst has changed from a rich air-fuel ratio to a lean air-fuel ratio. Then, the amount of fuel required for the air-fuel ratio of an air-fuel mixture, to be supplied into the internal combustion engine, to become rich is determined as a feed-forward fuel supply mount based on at least the intake volume, and a feedback correction amount is determined based on the air-fuel ratio upstream of the catalyst and the target air-fuel ratio such that the air-fuel ratio upstream of the catalyst matches the target air-fuel ratio. Subsequently, a final fuel supply amount is determined by correcting the feed-forward fuel supply amount using the feedback correction amount, and the same amount of fuel as the final fuel supply amount determined is supplied into the internal combustion engine.
Likewise, the target air-fuel ratio is set to the predetermined lean air-fuel ratio, a specific air-fuel ratio lean of the stoichiometric air-fuel ratio, when it is determined that the air-fuel ratio downstream of the catalyst has changed from a lean air-fuel ratio to a rich air-fuel ratio. Then, the amount of fuel required for the air-fuel ratio of the air-fuel mixture, to be supplied to the internal combustion engine, to become lean is determined as the feed-forward fuel supply mount based on at least the intake volume, and the feedback correction amount is determined based on the air-fuel ratio upstream of the catalyst and the target air-fuel ratio such that the air-fuel ratio upstream of the catalyst matches the target air-fuel ratio. Subsequently, the final fuel supply amount is determined by correcting the feed-forward fuel supply amount using the feedback correction amount, and the same amount of fuel as the final fuel supply amount determined is supplied into the internal combustion engine.
Namely, with the air-fuel ratio control system described above, when the air-fuel ratio is changed to the specific rich air-fuel ratio upon determining that the actual air-fuel ratio downstream of the catalyst has become lean, a corresponding feed-forward amount is determined, and the determined feed-forward amount is corrected by a feedback correction amount. Thus, the air-fuel ratio upstream of the catalyst can be immediately made rich without suffering from a delay in the feedback control during the time period where the catalyst is fulfilled with oxygen to its full capacity, which prevents the emission of a large amount of NOx.
Likewise, when the air-fuel ratio is changed to the specific lean air-fuel ratio upon determining that the actual air-fuel ratio downstream of the catalyst has become rich, a corresponding feed-forward amount is determined, and the determined feed-forward amount is corrected by the feedback correction amount. Thus, the air-fuel ratio upstream of the catalyst can be immediately made lean without suffering from a delay in the feedback control during the time period where no oxygen is stored in the catalyst, which avoids the emission of a large amount of unburned fuel.
Furthermore, with the air-fuel ratio control system described above, the feed-forward fuel supply amount is corrected using the feedback correction amount when changing the air-fuel ratio of the gas upstream of the catalyst to the predetermined rich or lean air-fuel ratio. Therefore, further accuracy can be achieved in controlling the air-fuel ratio of the gas upstream of the catalyst to the predetermined rich or lean air-fuel ratio.
In the air-fuel ratio control system described above, the feed-forward fuel supply amount may be a value which becomes largest right after the target air-fuel ratio has been switched and which decreases in time.
In this case, it is preferable that the fuel supply amount calculating means be adapted to calculate, as the feed-forward fuel supply amount, a fuel amount theoretically required for an air-fuel ratio of the air-fuel mixture, to be supplied into the internal combustion engine, to match the target air-fuel ratio on the basis of the value corresponding to the intake volume and the target air-fuel ratio.
According to this construction, the air-fuel ratio of the gas flowing into the catalyst can be made substantially equal to the target air-fuel ratio right after switching the target air-fuel ratio. This assures improved reliability in preventing an increase in the emissions, and enables the maximum storable oxygen amount of the catalyst, which is an element used for determining the degradation of the catalyst, to be obtained at the constant air-fuel ratio.
Also, it is preferable that the air-fuel ratio control system described above further include maximum storable oxygen amount calculating means which determines an amount of oxygen flowing into the catalyst from when the target air-fuel ratio is changed to the predetermined lean air-fuel ratio to when the air-fuel ratio detected by the downstream-side air-fuel ratio detecting means changes from a rich air-fuel ratio that is rich of the stoichiometric air-fuel ratio to a lean air-fuel ratio that is lean of the stoichiometric air-fuel ratio; and which calculates a maximum storable oxygen amount of the catalyst on the basis of that determined amount of oxygen, and/or it is preferable that the air-fuel ratio control system described above further include maximum storable oxygen amount calculating means which determines an amount of oxygen flowing into the catalyst from when the target air-fuel ratio is changed to the predetermined rich air-fuel ratio to when the air-fuel ratio detected by the downstream-side air-fuel ratio detecting means changes from a lean air-fuel ratio that is lean of the stoichiometric air-fuel ratio to a rich air-fuel ratio that is rich of the stoichiometric air-fuel ratio; and which calculates a maximum storable oxygen amount of the catalyst on the basis of the that determined amount of oxygen.
As described above, the air-fuel ratio upstream of the catalyst can be immediately made equal to the predetermined rich or lean air-fuel ratio in the above-described air-fuel ratio control system. Therefore, when the maximum storable oxygen amount is calculated during the time period from when switching the target air-fuel ratio to when a change in the air-fuel ratio downstream of the catalyst as a result of switching of the same target air-fuel ratio is detected, it is possible to perform that calculation of the maximum storable oxygen amount at a constant air-fuel ratio substantially equal to the predetermined rich or lean air-fuel ratio, so further accuracy can be achieved in determining the degradation of the catalyst.
An air-fuel ratio control method according to another aspect of the invention includes the steps of: obtaining a value corresponding to an intake volume of the internal combustion engine; detecting an air-fuel ratio upstream of the catalyst; detecting an air-fuel ratio downstream of the catalyst; setting a target air-fuel ratio to a predetermined rich air-fuel ratio that is rich of a stoichiometric air-fuel ratio when it is determined that the air-fuel ratio downstream of the catalyst has changed from a rich air-fuel ratio to a lean air-fuel ratio; setting the target air-fuel ratio to a predetermined lean air-fuel ratio that is lean of the stoichiometric air-fuel ratio when it is determined that the air-fuel ratio downstream of the catalyst has changed from a lean air-fuel ratio to a rich air-fuel ratio; calculating, as a feed-forward fuel supply amount, a fuel amount required for an air-fuel ratio of an air-fuel mixture, to be supplied to the internal combustion engine, to become rich of the stoichiometric air-fuel ratio on the basis of at least the intake volume when the target air-fuel ratio is set to the predetermined rich air-fuel ratio; calculating, as the feed-forward fuel supply amount, a fuel amount required for the air-fuel ratio of the air-fuel mixture, to be supplied to the internal combustion engine, to become lean of the stoichiometric air-fuel ratio on the basis of at least the intake volume when the target air-fuel ratio is set to the predetermined lean air-fuel ratio; calculating a feedback correction amount based on the air-fuel ratio detected by the upstream-side air-fuel ratio detecting means and the target air-fuel ratio such that the air-fuel ratio detected by the upstream-side air-fuel ratio detecting means matches the target air-fuel ratio; determining, as a final fuel supply amount, the feed-forward fuel supply amount corrected by the feedback correction amount; and supplying the internal combustion engine with the same amount of fuel as the final fuel supply amount.
In this method, the same effects and advantages as obtained with the air-fuel ratio control system according to the invention described above can be obtained.